This invention relates to improvements in techniques and apparatus for erasing information stored on magnetic storage media.
There are many applications in which it is necessary to erase information on a magnetic storage medium, such as a magnetic tape, or the like. It is known, for example, that magnetic tape or tracks on a magnetic disc can be erased or "degaussed" by applying magnetic fields of opposite polarity across the tape or across magnetic disc tracks. In some applications, such as where the information to be erased is unclassified data, voice, music, etc., the thoroughness of the erasing technique may not be particularly important. On the other hand, for applications wherein the information to be erased is classified data, it is very important that the data be completely erased. However, there has generally been a problem in the prior art in attaining reliable and complete erasure of information from magnetic storage media, particularly for high coercivity or so-called high energy media. The thoroughness of erasure, including removal of noise, is also important in applications where information having a high signal-to-noise ratio, for example a high quality video signal, is to be subsequently recorded on the magnetic storage medium.
The configuration of the particular magnetic storage medium to be erased is an important factor in determining whether or not a given technique may be suitable for achieving sufficient erasing of information. For example, in some prior art systems it is necessary to have erasing heads or coils in close proximity to strips or tracks of magnetic media which may not be readily accessible, due to such factors as the sealing of the magnetic medium in an enclosure or the shape or packaging configuration of the magnetic medium as compared to the design of a particular erasing apparatus.
A further factor that is significant is the time and effort needed to implement the erasing of magnetic media. Prior art techniques which erase a strip or track of tape at a time are usually slow, and may require periodic operator intervention or monitoring. Even some techniques that erase bulk tape tend to be unduly complicated, which increases the time and effort needed for the erasing process, and increases the probability of human error or equipment failure.
In U.S. Pat. No. 4,551,782, assigned to the same assignee as the present application, there is disclosed a technique for degaussing a reel of magnetic tape wherein the reel is simultaneously rotated on its axis and subjected to a transverse magnetic field substantially in the plane of the reel. The transverse magnetic field is applied first in one direction and then in the opposite direction. An axial magnetic field, substantially parallel to the axis of the reel, is then applied, first in one direction and then in the opposite direction. In the embodiment set forth, the axial field is applied using a large ring-shaped coil that surrounds the periphery of the reel. The magnetic fields are applied with successively decreasing amplitudes.
In the U.S. Pat. No. 3,023,280, two opposing degaussing heads each contain a pair of coils. Each degaussing head has has an E-lamination of magnetic material, and the coils of each head are wound in series and are on the outer legs of the E-laminations. The coils are energized with AC current. The opposing coils at one end of the degaussing heads are wound to produce magnetic fields that are aiding (to obtain a vertical magnetic field), and opposing coils at the other end of the degaussing heads are wound to produce magnetic fields that are bucking (to obtain a horizontal magnetic field). A rotating reel of tape is passed between the degaussing heads. The tape is exposed to decreasing magnetic fields as it is passed through the fringe fields, this being done by shunting the fields with a sheet of iron. In addition to requiring four coils, the power in this system is shared by the two pairs of coils, and this limits the magnetic field strengths, particularly for the important field in the horizontal direction, which already tends to have a smaller field amplitude than the vertical field. Further, with the different field orientations occurring at once, the fields are not well defined in the desired vertical and horizontal directions, and it is necessary to ensure that all of the tape passes through both fields.
U.S. Pat. No. 3,938,011 discloses a tape degausser in which AC or DC is applied to upper and lower demagnetizing coils. The magnetism of the demagnetizing units is repeatedly reversed in polarity, with diminishing magnitude, to achieve degaussing.
In the U.S. Pat. No. 4,384,313 there is disclosed a process for demagnetization which uses alternating magnetic fields and capacitance to obtain resonance.
Reference can also be made to the following U.S. Pat. Nos.: 245,275, 733,637, 2,240,749, 2,786,970, 3,143,689, 3,303,398, 3,321,586, 3,588,623, 3,609,465, 3,619,729, 3,647,990, 3,651,382, 3,655,924, 3,657,638, 3,723,668, 3,810,230, 3,938,011, 3,958,273, 4,135,219, 4,157,581, 4,306,269, 4,346,426, 4,359,765, 4,360,854, 4,384,313, 4,458,178, 4,402,032, and 4,466,027.
Prior art techniques, such as those described in the patents noted above, have employed certain features to advantage in the degaussing of bulk magnetic tape reels, including: use of magnetic fields of different orientations; gradual reduction of the fields; and rotation of the tape during field application. Also, resonant circuits have been used to increase the power delivered to degaussing coils. However, the complexity and cost of equipment needed to implement these features has been problematic. The convenience and reliability of the degaussing procedure is another factor which comes into play. For example, equipment that requires undue handling of the magnetic tape reel (or undue movement by degaussing heads) during the degaussing proceedure, either manual or by mechanical means, has inherent disadvantage. Finally, the completeness of degaussing has still not met the most stringent requirements for certain applications.
It is among the objects of the present invention to provide a degaussing method and apparatus wherein controlled magnetic field components of sufficient strength can be applied to a reel of magnetic tape at specified orientations, without the need for large input power, complex or expensive coil arrays, or complex mechanical means for moving the magnetic tape or the coils.